Okay, so we know that the most popular Game of Thrones fan theory is official, because, unlike Siri, HBO said so. Rhaegar Targaryen + Lyanna Stark = Jon (or Jaehaerys) Snow. But — and this is an important point that could have saved lives — why didn’t Ned tell Catelyn about Jon’s heritage? Why did he continue to spread the lie that Jon is his “illegitimate bastard son” with a servant girl named Wylla? Seems like it would have prevented a lot of heartbreak — Catelyn never considered Jon part of the family, and Jon could have used a mother’s warm embrace. Why do you think he looks so gloomy?
Perhaps George R.R. Martin will get around to answering this in The Winds of Winter or A Dream of Spring (probably not, though, especially if the books aren’t coming out until, like, 2047 and 2071). But a Science Fiction & Fantasy forum user, the aptly named Aegon, came up with a solid explanation.
Because Eddard did not trust Catelyn at the time enough to tell her about a secret that could lead to murder of Jon and destruction of House Stark. Later the topic just never came up and both of them decided not to bring it up and enjoy a loving marital life. Nothing was to be gained except guaranteed security of Jon. (Via)
Imagine your life partner did something bad. Maybe not cheat on you with a servant, but something that hurt your feelings. You have two choices: You can either leave him or her, or you can work through your issues, forgive what happened, and go back to living in happy married bliss. That’s more or less what Ned and Cat decided, although she was reminded of what her otherwise perfect husband did — even if he didn’t actually do anything — every time she saw Jon; no wonder she kept him at arm’s length. Anyway, “Aegon” also broke down Ned and Cat’s relationship history, which the show doesn’t get into very much. But his main conclusion is, “Eddard took his vows very seriously and he promised his sister to keep Jon safe. He endured jeering, mocking, and confrontations with his wife, but he never put Jon’s security at risk by telling his true identity to anyone. Some things are better left buried.”
Good thing for the North, that’s a lesson Melisandre never learned.